1. Field of the Invention
The present invention relates to an overlaying alloy advantageously usable for overlaying an engine valve of diesel engines using light oil as a fuel, engines using compressed natural gas (CNG) or liquified petroleum gas (LPG) as a fuel, or other engines in which the oxidizing power of the combustion atmosphere is weaker than that of gasoline engines.
2. Description of the Related Art
FIGS. 1A and 1B show an engine valve 1 having a valve face 2, which is required to have good properties including wear resistance, non-damaging property, heat resistance and thermal shock resistance over a wide temperature range from room temperature to elevated temperatures. The term "non-damaging property" means that an overlaid part does not significantly damage a counterpart which is in butting relationship with the overlaid part. The term "damaging property" means the intensity at which an overlaid part damages a counterpart. The heat resisting steels commonly used as a material of engine valves cannot satisfactorily provide these properties, and therefore, alloys having these properties are overlaid on the valve face to impart the required properties.
When a gasoline engine is operated, a combustion product is formed on an overlaid layer of the engine parts such as a piston ring, a valve face, and a valve seat and suppresses wear which would otherwise occur in such parts. The combustion product is substantially composed of oxides of the alloy components of the overlaid layer and acts as a protective film providing solid lubrication.
However, diesel engines using light oil as a fuel and engines using CNG, LPG or other gases as a fuel provide a combustion atmosphere having a weak oxidizing power such that the combustion product substantially composed of oxides hardly forms on the surface of the engine parts. As a result, no effective lubrication occurs and an overlaid layer of a wear resistant material only causes metal-to-metal contact between the parts leading to significant wear.
A known solution to the above problem is adding Cr and/or Mo in an alloy composing the parts of diesel engines and engines using CNG or LPG as a fuel to allow Cr oxides and/or Mo oxides to be formed on the surface of the alloy and to provide solid lubrication.
For example, Japanese Unexamined Patent Publication (Kokai) No. 59-100263 discloses an iron-based alloy as a material for thermal-spraying the pistons of diesel engines, which alloy consists of 1-4 wt % C, 10-30 wt % Cr, 10-30 wt % Mo, 1-5 wt % Nb, 20-40 wt % Co, 2-15 wt % Ni and the balance of Fe and unavoidable impurities.
Stellite alloy is generally used as an alloy for overlaying the valve face of the engine valves of engines using CNG or LPG as a fuel, which alloy consists of 1.2 wt % C, 29.8 wt % Cr, 0.01 wt % Mn, 1.35 wt % Si, 4.7 wt % W and the balance of Co and unavoidable impurities.
However, the above recited conventional overlaying alloys cannot satisfactorily provide non-damaging property and good wear resistance at practically acceptable level, and therefore, an overlaying alloy having an improved non-damaging property and wear resistance is desired.